In Formula: a(n) = 3*(a(n-1)-1) + 2^n. should read: a(n) = 3*a(n-1) + 2^(n-1) ie. a(0)=0 a(1)=1 a(2)=3.1+2=5 a(3)=3.5+4=19 etc... Jon Perry perry at globalnet.co.uk http://www.users.globalnet.co.uk/~perry/maths BrainBench MVP for HTML and JavaScript http://www.brainbench.com